Restart your GNOME shell (Alt-F2 r is one way) and enable the
extension using gnome-tweak-tool (install it if not present).

If the extension does not install, check the version number in
metadate.json. You may have to change it to work with your
particular version of the GNOME Shell. If this does not fix
the problem, use Looking Glass (Alt-F2 lg) to see what the
error message is.

Current Version

Release 0.4.3

Other Info

SFB parse bookmarks backup json file that Firefox stores usually in the dir:

$HOME/.mozilla/firefox/<profile dir>/bookmarkbackups/

SFP try to retrieve the last json backup, but you can suggest the correct path by two way: